The International Affective Picture System! IAPS for short.
These pictures, they make you feel things. In a standardized way.
See this guy here wanted to study emotion, so he got this set of pictures together and showed them to people, and he had them rate how good/bad the pictures made them feel, and also how intensely the pictures made them feel that way. And then he averaged these scores.
So now if I want to make you feel happy for the purposes of studying your brains, I can pull out an IAPS picture with a high happiness rating, like bunnies nibbling on cut flowers. The most intense happy pictures are pretty much softcore porn.
But if I want to make you feel unhappy, I can show you a picture of a pointed gun, or a starving child, or a dead kitty. And if I want to make you intensely unhappy, I can show you a mutilated face.
I have spent a lot of my time looking at emotional pictures lately.